Okay, my engine builder and machinist figured out that my engine is close to 9.3:1 compression. I don't have timing bumped up, it's at 6 degrees as the book says it should be. So, it should have no problem running on 91 octane right. THe local gas stations only really carry 91 as their highest. But, on last weekend, I hit a gas station that sold 93 and another that sold 94. I filled the tank with the 93 and when it needed it again, I put the 94 in.

If my car only needs 91 (as per low compression ratio) , why does it run so much better and smoother on the 93/94 stuff? It idled better, it ran smoother and it felt like it had more power. This all couldn't have been in my head, could it?

 

Best thing you can do is hook it up to a scanner and see what the lowest octane you can run is without getting any knock counts.

You could also test it on a drag strip, go one day with 93/94 in the car, and another (with at least similar outside conditions) with 91.

 

Yes it could....but.......your engine may be responding to a more efficient burn with the higher octane. Your computer will adjust to the octane being burned, either higher or lower. If you are pleased with the results provided by the higher octane, and the increased cost is within reason, go for it.

 

This is true...it happens all the time, especially after "mods"...the butt dyno gives you false results sometimes.

This, however, is not true at all. First of all, the only time the ECM will adjust to the octane being burned is if it has to pull timing due to knock counts. It will never put in any more timing than what is in the timing tables in the PROM. It has NO WAY of knowing, nor does it care, what octane fuel is being put into the motor. Secondly, the octane level has NOTHING to do with the efficiency of burn. With a couple of exceptions found in race fuels, the ONLY THING that octane does is change the flash point of the fuel with respect to temperature and pressure. The octane DOES NOT change the efficiency of the burn, nor does it add power in any way shape or form, with 2 exceptions. 1) some race fuels use an oxygenate (oxygen-carrying molecule in the fuel) to raise octane, and 2) if you are comparing two fuels where one has a low enough octane level to trigger the knock sensor, which will subsequently pull out timing, thus reducing power.

Getting back to the original question, you should be able to run 91 octane without ANY problems on a 9.3:1 CR. You could also run 89, if you so chose, assuming your engine is in good running order (rings in good shape, valve seals not leaking, engine not running hot, etc, etc).


[Modified by CorvetteZ51Racer, 2:14 AM 5/24/2003]

 

"BUTT DYNO" i love that term! we all have one but why is so mistakenly wrong and unreliable?! :jester

 

I have recently replaced the knock sensor due to the fact I thought I was receiving false knock. I had it on the dyno at Agostino Racing and the dyno guy backed out of it at around 3500 RPMs saying it was detonating and something was screwing up with the timing. He also asked me about the condition of the plugs and wires. Over the winter during one of my engine startups ( I started it every 2 weeks and let it get to operating temp) I noticed that the spark was arcing from the boot to the header. So, at the start of the fall, I had my mechanic put in NGK TR6's and Taylor Spiro Pro 8mm wires, and I noticed an improvement. Like I mentioned, I had also changed the knock sensor. I believe it's running a lot better now, but haven't made my way back to the dyno yet. The engine does seem to have more power and smoother acceleration throughout the RPM range with the higher octane.

The dyno guy said it was detonating pretty bad and sounded like it was 12:1 comp. ratio, but I told him on paper, it should be like 9.5 or under. My mechanic said it should be 9.3. I wonder if my mechanic was wrong about my comp. ratio. So, I had him run a compression check, and I believe he said they were all around the 165 to 180 range. Can't remember exactly.

What is the compression ratio that would detonate and pull timing on 91 octane and not 94? Octane vs. compression ranges, anyone?

 

On an L98, right around the 10-10.2:1 mark. My old motor was 10.4:1, and if I ran anything under 93, it detonated like crazy. I could always tell when a gas station was mixing its higher octane tanks.